This invention relates to an improved process for forming large optical performs for the production of optical fibers. The process provides large diameter optical preforms which are made to possess step index, single mode or graded index profiles. Such characteristics enable the production of optical fiber cables exhibiting reliable operating characteristics.
There has been a continuous search in the prior art for the economical and mass production of fiber optic cables for use in optical communications systems.
Thus, the prior art considered and describes techniques as "soot" deposition or hydrolysis wherein a gas vapor mixture is hydrolyzed by a flame to form a glass precursor particulate. The particulate is then deposited on a rotating glass rod serving as a mandrel. The soot is deposited upon the mandrel in a perpendicular direction to provide successive layers of constant radius or to provide preforms with radial graduations by varying the dopant concentration in successive passes of the burner flame. The mandrel is removed and a cylindrical preform is collapsed to a solid rod and then drawn into a fiber. This process is shown and discussed in U.S. Pat. No. 3,826,560 and U.S. Pat. No. 3,823,995.
Other techniques as in U.S. Pat. No. 3,614,197 describe processes for continuously forming a fiber optic cable by using a multi-stepped funnel-shaped vessel to form a solid glass rod which is then heated and drawn into a cable.
In any event, there is a desire to provide a solid optical preform and then draw or process the same into a fiber optic cable. Both the continuous process and the preform approach have inherent benefits in the mass production of such cables.
Hence, U.S. Pat. No. 3,966,446 entitled AXIAL FABRICATION OF OPTICAL FIBERS issued on June 24, 1976 discusses a technique for providing an optical preform. The optical preform is fabricated by the axial deposition from a direction along the preformed axis as opposed to radial deposition from a direction perpendicular to the preformed axis. The technique does not require a mandrel and thus avoids the collapse of a cylindrical preform prior to drawing.
The preforms thus provided in the above noted patent possess longitudinal gradations in the index of refraction and thus serve to enhance certain types of mode conversions.
In any event, there is a need to provide large optical preforms which then can be drawn into elongated optical fiber cables. There is a further need to provide an optical preform which can exhibit step, single mode or graded index profiles to enable the resultant cable to be used to more efficiently transmit optical information in the form of digital or other signals.
It is known that fiber cables which possess a single mode of operation alleviate mode dispersion problems. It has been a problem to produce reliable cables employing single mode operation in that the prior art techniques could not adequately control the composition of the cable. Thus, many cables employ a multi-mode operation in using radial gradations in the index of refraction. In these cables the difference in velocity from mode to mode compensates for the different path lengths and results in a relatively equal transversal time for all modes.
In any event, it is clear that in order to efficiently employ a single mode or a multi-mode operation, one must carefully and accurately control the fabrication of the cable to assure that the same is consistent in formulation and hence, possesses repeatable and reliable operating characteristics.